Sound propagation across walls is often a nuisance. In addition, privacy concerns can arise from overhearing conversations through thin walls. City sounds keep people awake. Loud music can drive parents to distraction, even through thicker walls. Passive sound reduction techniques, including thickening walls and using special sound dampening materials, can only reduce or muffle sounds so much. Active sound reduction techniques are desired but are sometimes expensive or ineffective. Methods and systems for modifying sound waves passing through a wall are needed.